Change to use spread syntax for shallow clones

Closes GH-8.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Author: remcohaszing

lib/index.js

@@ -281,11 +281,9 @@ export function buildJsx(tree, options) {
 
       /** @type {MemberExpression | Literal | Identifier} */
       let name
-      /** @type {Array<Property>} */
-      let fields = []
+      /** @type {Array<Property | SpreadElement>} */
+      const fields = []
       /** @type {Array<Expression>} */
-      const objects = []
-      /** @type {Array<Expression | SpreadElement>} */
       let parameters = []
       /** @type {Expression | undefined} */
       let key
@@ -314,12 +312,12 @@ export function buildJsx(tree, options) {
           const attribute = attributes[index]
 
           if (attribute.type === 'JSXSpreadAttribute') {
-            if (fields.length > 0) {
-              objects.push({type: 'ObjectExpression', properties: fields})
-              fields = []
+            if (attribute.argument.type === 'ObjectExpression') {
+              fields.push(...attribute.argument.properties)
+            } else {
+              fields.push({type: 'SpreadElement', argument: attribute.argument})
             }
 
-            objects.push(attribute.argument)
             spread = true
           } else {
             const prop = toProperty(attribute)
@@ -373,33 +371,11 @@ export function buildJsx(tree, options) {
         parameters = children
       }
 
-      if (fields.length > 0) {
-        objects.push({type: 'ObjectExpression', properties: fields})
-      }
-
-      /** @type {Expression | undefined} */
-      let props
       /** @type {MemberExpression | Literal | Identifier} */
       let callee
 
-      if (objects.length > 1) {
-        // Don’t mutate the first object, shallow clone instead.
-        if (objects[0].type !== 'ObjectExpression') {
-          objects.unshift({type: 'ObjectExpression', properties: []})
-        }
-
-        props = {
-          type: 'CallExpression',
-          callee: toMemberExpression('Object.assign'),
-          arguments: objects,
-          optional: false
-        }
-      } else if (objects.length > 0) {
-        props = objects[0]
-      }
-
       if (automatic) {
-        parameters.push(props || {type: 'ObjectExpression', properties: []})
+        parameters.push({type: 'ObjectExpression', properties: fields})
 
         if (key) {
           parameters.push(key)
@@ -470,9 +446,10 @@ export function buildJsx(tree, options) {
       }
       // Classic.
       else {
-        // There are props or children.
-        if (props || parameters.length > 0) {
-          parameters.unshift(props || {type: 'Literal', value: null})
+        if (fields.length > 0) {
+          parameters.unshift({type: 'ObjectExpression', properties: fields})
+        } else if (parameters.length > 0) {
+          parameters.unshift({type: 'Literal', value: null})
         }
 
         callee = toMemberExpression(